In Cushing's disease, where there's a pituitary adenoma secreting ACTH, I think the excess ACTH causes hyperplasia of the zonas fasiculata and reticularis. The zona reticularis will now secrete more androgens, thus causing hirsutism and amenhorrea. I think the amenorrhea is a result of the increased androgen: estrogen ratio, though I'm not entirely sure about this. Perhaps there's suppression of LH release secondary to increased androgen feedback...?